1. Field of the Invention
The invention relates to laser spectrometers and, more particularly, to a laser spectrometer and method for measuring the concentration of a gas component in a measurement gas.
2. Description of the Related Art
Laser spectrometers are particularly used for optical gas analysis in process measurement technology. Here, a laser diode generates light in the infrared range, which is guided through the process gas to be measured (the measurement gas) and is subsequently detected. The wavelength of the light is tuned to a specific absorption line of the gas component respectively to be measured, the laser diode periodically sampling the absorption line. To this end, the laser diode is driven periodically with a ramp-shaped or triangular (increasing and decreasing ramp) current signal. The concentration of the gas component of interest can be determined from the absorption detected at the position of the absorption line.
The intensity and wavelength of the light generated are nonlinear functions of the injection current and of the operating temperature of the laser diode. As a result, wavelength referencing is necessary in many cases. To this end, a reference gas is additionally introduced in a known concentration into the light path, and an absorption line of the reference gas is measured. The temperature of the laser diode can then be regulated via the position of the absorption line of the reference gas, such that the absorption line of the gas compared to be measured always lies at a particular position of the ramp of the current signal. In this case, the current ramp must be large enough for the laser diode sampling range resulting therefrom to cover both the absorption line of the gas component to be measured and that of the reference gas.
When shining through the measurement gas and reference gas, besides the wavelength-dependent absorption by infrared-active gas components, wavelength-independent absorption also takes place by optical components (e.g., windows) or aerosols (e.g., smoke particles). This makes normalization of the measurement necessary. To this end, the laser diode can be driven regularly with at least one burst current signal, the amplitude of which lies outside the value range of the ramp-shaped or triangular current signal, so that the light wavelengths generated with the burst current signal lie outside the wavelength ranges of the absorption lines of the gas components to be measured and other infrared-active gas components. This makes it possible to normalize the light intensity detected at the position of the absorption line to be measured, by division by the light intensity detected at the position of the burst current signal (EP 2 072 979 A1).
As explained above, in contemporary laser spectrometers a wavelength range that covers both the absorption lines of the gas components to be measured and the absorption lines for the wavelength referencing is sampled. In addition, a time window is required for the normalization of the measurement. Each sampling period therefore claims much more time than is necessary for the detection of a single absorption line. The time resolution of the measurement, in the case of rapidly varying gas concentrations, is thereby limited.